Installation for etching objects

ABSTRACT

An installation for etching objects comprises at least one etching machine, in which metal is etched from the treated objects, the etching medium being enriched with metal. The etching medium is regenerated again in several electrolytic cells by the removal of metal. The electrolytic cells are brought into operation successively in adaptation to different instantaneous etching capacities of the etching machine (&#34;loads&#34;). This takes place by means of a device, which integrates the quantity of the enriched etching medium removed from the etching machine, over a predetermined period of time. A certain limit value of this integral is associated with each electrolytic cell; if this limit value is exceeded, the corresponding electrolytic cell is activated. In this way, the total capacity of the respective electrolytic cells in operation is adapted to the instantaneous load of the etching machine.

The invention relates to an installation for etching objects, inparticular printed circuit boards, with

a) at least one etching machine, in which metal is etched from theobjects, the etching medium being enriched with metal;

b) at least one electrolytic cell, in which enriched etching medium isdepleted;

c) at least one electronic control circuit, which controls the exchangeof etching medium between the etching machine and the electrolytic cellso that the density of the etching medium in the etching machine issubstantially constant.

In known installations of this type, the capacity of the electrolyticcells is adapted to the maximum capacity of the etching machine, thecontrol system always setting the electrolytic cell in operation whenthe density of the etching medium in the etching machine exceeds acertain value. These etching installations sometimes also containseveral electrolytic cells connected in parallel, however this is formanufacturing reasons, since then a uniform type of electrolytic cellcan be used for etching machines of the most varied capacity. All theelectrolytic cells are always operated as a unit. In these known etchinginstallations, the following is a drawback: if etching machines, whichhave a larger capacity, are operated with a low instantaneous etchingcapacity ("load"), the connected electrolytic cells very frequentlyswitch on and off. However, such non-continuous operation of theelectrolytic cells leads to a poor formation of the metal layer on thecathodes. In addition, the control of the density of the etching mediumto the desired value for each new switching-on of the electrolytic cellsonly begins late, since the latter are normally put out of operation bydischarge of the etching medium and a considerable time lapses untilre-filling of the etching medium. The considerable control fluctuationsof the density of the etching medium in the etching machine connectedtherewith have a disadvantageous effect on the etching result.

It is the object of the present invention to design an etchinginstallation of the aforementioned type so that the switching-on of theelectrolytic cells takes place with better adaptation to theinstantaneous load of the etching machine and the frequency ofswitching-on and off of the electrolytic cells is reduced.

The solution of this object according to the invention is characterisedby

c) a device, which

ea) measures the quantity of the enriched etching medium removed fromthe etching machine and integrates it over a predetermined time;

eb) in which as many limit values of different quantity for the integralare stored, as there are electrolytic cells in the installation;

ec) on exceeding each limit value of the integral a (further)electrolytic cell is set in operation or on falling below each limitvalue of the integral a (further) electrolytic cell is put out ofoperation;

so that the more electrolytic cells are in operation, the more enrichedetching medium is removed from the etching machine in the predeterminedtime.

Thus, according to the invention, several electrolytic cells of lowercapacity are used intentionally, not solely for manufacturing reasons,as was the case in the prior art. These electrolytic cells are not allconnected as a unit; on the contrary, the number of electrolytic cellswhich are in operation at any time, is determined by the quantity ofetching medium removed from the etching machine over a predeterminedintegration time. With a very small quantity of etching medium, thussolely a single electrolytic cell is in operation, which is thenconnected to a further electrolytic cell, when the amount of etchingmedium exceeds the first limit value of the integral and so on. This hasthe result that the frequency of switching the electrolytic cells on andoff is considerably reduced. The build-up of the metal layer on thecathodes is considerably better than in the prior art. Considerablylower control fluctuations in the density control of the etching mediumin the etching machine also result, which in turn leads to betteretching results.

In a particularly preferred embodiment, the device which takes over thecontrol of the electrolytic cells, has a mechanical construction. Inthis case, it comprises a buffer tank, into which the enriched etchingmedium removed from the etching machine is introduced, the buffer tankin each case being connected by way of a line, in which a pump islocated, to each electrolytic cell and a different level of the fillingheight in the buffer tank being associated with each of these pumps sothat it is operative solely after exceeding this level. This mechanicalrealisation of the idea according to the invention is based on thefollowing consideration: all pumps, which convey etching medium from thebuffer tank to the various electrolytic cells, have a certain, limiteddischarge capacity co-ordinated with the capacity of the electrolyticcell. As long as the discharge capacity of the first pump, whichconnects the buffer tank to the first electrolytic cell, is adequate,the level of the filling height in the buffer tank never rises above acertain value. However, if the output of etching medium from the etchingmachine is greater than the first pump can remove, the level of thefilling height in the buffer tank rises until the second pump becomesoperative on reaching the next limit value. Now both pumps work togetherwith the sum of their delivery capacities. A further rise of the etchingmedium in the buffer tank takes place solely if the quantity of etchingmedium supplied per unit time by the etching machine exceeds thecombined delivery capacity of the first two pumps; then, a third pumppossibly starts up, which conveys etching medium to the thirdelectrolytic cell and so on.

With this embodiment, two methods of construction are again possible: inthe first, level sensors are provided, which monitor when various limitvalues of the filling height in the buffer tank are reached and start orstop the corresponding pumps in accordance therewith.

It is simpler, if the lines, by which the etching medium is removed fromthe buffer tank, terminate at the height of the various levels of thefilling heights and if the pumps operate continuously. In this case, thepumps, which are not necessary for discharging the etching mediumsupplied per unit time to the buffer tank, are simply therefore out ofoperation (and likewise the connected electrolytic cells), because theassociated removal line does not reach into the etching medium in thebuffer tank.

It is particularly appropriate if a buffer tank is likewise located inthe line, by which the depleted etching medium is conveyed back from theelectrolytic cells to the etching machine. Together with the buffer tankalready mentioned above, which serves as a "sensor" for the quantity ofetching medium removed per unit time from the etching machine, thisfurther buffer tank decouples the control systems, which on the one handin the etching machine and on the other hand in the electrolytic cellsensure a constant density of the etching medium located there.

For saving energy, it may be provided that the etching medium suppliedto one buffer tank from the etching machine and the etching mediumsupplied from the other buffer tank to the etching machine are passedthrough a heat exchanger.

If one uses buffer tanks, as proposed above, then the evaporation ofliquid is increased, which can also be ascertained in otherinstallations, even if in smaller quantities. To supplement the loss ofliquid, it is recommended that a water control unit is provided, whichkeeps the sum of the filling heights in the various sumps, containersand tanks of the etching installation constant by the addition of freshwater.

As regards circuit techniques and apparatus, that embodiment isparticularly favourable, in which

a) the filling heights in the buffer tanks are monitored by levelsensors, which are connected to the water control unit;

b) the sum of the filling heights in the buffer tanks is kept constantby the addition of fresh water;

c) the filling heights in the other parts of the installation are keptconstant independent of the addition of fresh water.

Generally the above condition c) is fulfilled anyway by the method ofconstruction of the etching machine and the electrolytic cells. Thus, inthis case, solely the filling heights in the two buffer tanks need to bemonitored, to which the corresponding quantity of fresh water is thensupplied.

Now in order to avoid that the supply of fresh water to the buffer tanksleads to undesirable dilutions of the etching medium contained therein,the addition of fresh water to each of the buffer tanks advantageouslytakes place in relation to the filling heights of these buffer tanks.The buffer tank which is more full thus contains a greater quantity offresh water than the buffer tank which is more empty, so that thedilution by fresh water in both buffer tanks is approximately the same.

Whereas, in the embodiments of the invention described above in detail,the device which undertakes the control of the electrolytic cells, wasconstructed mechanically, it is also possible that this device isconstructed electrically and comprises:

ed) a flow meter;

ef) an integrator, which integrates the output signal of the flow meterover the predetermined period of time;

eg) a memory, in which the various limit values of the integral arestored;

eh) a comparator, which compares the output signal of the integratorwith the limit value stored in the memory and on reaching one of theselimit values brings the associated electrolytic cell into or out ofoperation.

One embodiment of the invention will be described hereafter in detailwith reference to the drawings; the single FIGURE shows diagrammaticallyan installation for etching objects.

The installation for etching objects illustrated in the drawingcomprises as its main components an etching machine 1, a dosing unit 2,a first buffer tank 3, a second buffer tank 4, three electrolytic cells5a, 5b, 5c as well as three storage tanks 6a, 6b, 6c for etching mediumdischarged from the electrolytic cells 5a, 5b, 5c. The electrolyticcells 5b, 5c, the associated storage tanks 6b, 6c and the connectinglines and other devices are indicated solely diagrammatically by boxes;they correspond to the electrolytic cell 5a and the associated storagetank 6a as well as to the corresponding connecting lines and otherdevices shown in detail.

The construction of the etching machine 1 is basically known: theobjects 7 to be etched are moved by the continuous method from an inlet8 to an outlet 9 of the etching machine on a roller conveying system 10.They thus pass an upper nozzle arrangement 11 as well as a lower nozzlearrangement 12, by which they are sprayed with etching medium. Thelatter is supplied by a pump 13, which is connected on the suction sideto the sump 14 of the etching machine, to the nozzle arrangements 11,12. The etching medium drips from the objects 7 to be etched back intothe sump, in which case it changes its chemical composition on accountof the etching process and due to evaporation processes.

A dosing unit 2 is provided for monitoring and controlling the chemicalcomposition of the etching medium in the etching machine 1. The sump 14of the etching machine 1 is connected by way of a connecting line 15 toa tank 16 of the dosing unit 2. A pump 17 removes etching mediumcontinuously from the tank 16 and returns the latter in the circuit byway of a ph-meter 18 and a density-measuring device 19 to the tank 16.The pressure side of the pump 17 is furthermore connected to twoinjectors 20, 21, in which NH₃ is mixed with the flowing etching medium.

In the left-hand injector 20 in the drawing, NH₃ is added to the flowingetching medium, which NH₃ originates from a storage tank and the flow ofwhich is determined by a solenoid valve 22. The solenoid valve 22 is inthis case controlled electrically by the ph-meter 18. A minimum pH valueof the etching medium in the etching machine 1 is thus ensured by meansof the ph-meter 18 due to the addition of NH₃ by way of the solenoidvalve 22.

By way of the right-hand injector 21 in the drawing, the gas sucked fromthe electrolytic cells 5a, 5b, 5c, which contains substantially ammonia,is returned to the etching medium. In this way, the evaporation lossesof NH₃ are kept small and environmental problems are reduced.

By means of the above-mentioned density-measuring device 19, the densityof the etching medium in the etching machine 1, which without specialprecautions was increased by the metal etched from the objects 7 (in thecase of printed circuit boards generally copper), is kept at a constantvalue. This takes place in the following manner:

The left-hand buffer tank 3 in the drawing contains a supply of etchingmedium, which was supplied by the electrolytic cells 5a, 5b, 5c in amanner described hereafter. On the other hand, the right-hand buffertank 4 in the drawing contains etching medium of higher density,enriched with copper, which will be supplied for depletion to theelectrolytic cells 5a, 5b, 5c in a manner likewise to be describedhereafter.

A pump 23 is connected on the suction side by way of a line 24 to thebuffer tank 3. It conveys the depleted etching medium removed from thebuffer tank 3 through a heat exchanger 25 into the sump 14 of theetching machine 1. A further pump 26 is connected by way of a line 27 onthe suction side to the sump 14 of the etching machine 1. The openingpoint of the line 27 is located at a height which corresponds to theoperating level of the sump 14 in the etching machine 1. The pump 26likewise conveys the etching medium removed from the sump 14 of theetching machine 1 through the heat exchanger 25, where a heat exchangetakes place between the etching medium supplied to the sump and theetching medium removed from the sump 14. The etching medium conveyed bythe pump 26 then flows from the heat exchanger 25 into the second buffertank 4, in which, as mentioned above, etching medium enriched withcopper is located.

The pumps 23 and 26 are connected to each other electrically or--asillustrated--mechanically by a common motor. The arrangement is suchthat both pumps 23, 26 are always operated simultaneously, the deliverycapacity of the pump 26 always being kept somewhat higher than thedelivery capacity of the pump 23. In this way it is ensured that theoperating level of the etching medium in the sump 14 of the etchingmachine 1 is always determined by the opening point of the line 27 intothe sump 14.

The right-hand buffer tank 4 in the drawing is connected by lines 28a,28b, 28c to the suction side of pumps 29a, 29b, 29c, which are connectedon the pressure side to the sumps 32 of the electrolytic cells 5a, 5b,5c.

The overflows 34 of the electrolytic cells 5a, 5b, 5c, from which thedepleted etching medium flows, are connected by way of a line 35 to theleft-hand first buffer tank 3 in the drawing. A further line 36, inwhich a solenoid valve 37 is connected, leads from the overflow 34 ofeach electrolytic cell 5a, 5b, 5c into the associated storage tank 6a,6b, 6c. The sump 38 of each storage tank 6a, 6b, 6c is connected by wayof a line 39 to a pump 40, which supplies the etching medium removedfrom the sump 38 by way of a flow meter 41, with which a non-returnvalve 42 is connected in parallel, to the sump 32 of the electrolyticcell 5a, 5b, 5c. The pump 40 is also connected on the pressure side to asolenoid valve 43, which controls the flow path to a density-measuringdevice 44. The etching medium flowing through the density-measuringdevice 44 is returned to the storage tanks 6a, 6b, 6c.

Located in parallel with the density-measuring device 44 is a hydroxidefilter 45, the flow through which can be initiated if required by meansof a valve 46.

Electrolytic cells 5a, 5b, 5c and storage tanks 6a, 6b, 6c are operatedand controlled in the following manner:

First of all it is assumed that the delivery capacity of the pump 29aassociated with the first electrolytic cell 5a is adequate fordischarging the quantity of etching medium integrated over apredetermined period of time, which etching medium flows into the buffertank 4. Then the filling height in the buffer tank 4 remains at theheight which corresponds to the opening point of the line 28a into thebuffer tank. The other pumps 29b, 29c and the electrolytic cells 5b, 5cassociated therewith are not in operation and first of all may bedisregarded.

At the beginning of operation, the electrolytic cell 5a must be filledwith etching medium from the storage tank 6a. This takes place by meansof the pump 40. When the electrolytic cell 5a has reached its fillinglevel, this is ascertained by a level sensor 47, which opens thesolenoid valve 43. This releases a flow bypass, which reduces the flowof etching medium from the storage tank 6a into the electrolytic cell 5ato the amount necessary during continuous operation. A major part of theetching medium conveyed by the pump 40 now flows through thedensity-measuring device 44 and through the hydroxide filter 45 back tothe storage tank 6a.

Normally the solenoid valve 37 is open. This means the etching medium iscontinuously circulated by the pump 40 by way of the electrolytic cell5a, its overflow 34 and the solenoid valve 37. However, if etchingmedium passes from the buffer tank 4 into the sump 32 of theelectrolytic cell 5a, the level in the sump 38 of the storage tank 6arises. A level switch 48 records the rise of the liquid level in thesump 38 and closes the solenoid valve 37. Depleted etching medium nowflows by way of the line 35 into the buffer tank 3.

The density-measuring device 44 monitors the copper content of theetching medium circulated by the pump 40. If this copper content dropsbelow a predetermined value, for example below 30 g/l, then the pump 29is set in operation. On account of the above-described operations, inthis case a corresponding quantity of etching medium is removed from theelectrolytic cell 5a and supplied to the buffer tank 3. The supply ofenriched etching medium from the buffer tank 4 increases the density ofthe etching medium in the electrolytic cell 5a until thedensity-measuring device 44 once more stops the pump 29.

The electrolytic cell 5a is put out of operation if the buffer tank 4 isempty. This takes place by switching-off the pump 40. Consequently thecontents of the electrolytic cell 5a flow back by way of thedensity-measuring device 41 and mainly by way of the non-return valve42, line 39 and pump 40 into the storage tank 6a. However, theelectrolytic cell 5a remains alive.

For safety reasons, the electrolytic cell 5a is moreover always shutdown when the density of the etching medium contained therein fallsbelow a second value, which is below the above-mentioned control point.

The entire, above-described installation clearly contains two controlsystems, which are isolated from each other by the two buffer tanks 3,4:

The density-measuring device 19 operating in the first control circuitensures a constant density of the etching medium in the etchingmachine 1. The constant density is brought about by the supply ofdepleted etching medium from the buffer tank 3 or by the discharge ofenriched etching medium into the buffer tank 4. Owing to the existenceof the buffer tanks 3, 4, independently of the respective function ofthe electrolytic cell 5a, depleted etching medium or space for enrichedetching medium is always available. The first control system, whichcontains the density-measuring device 19 as the "core", can thus operatecompletely "autonomously".

The second control system contains the density-measuring device 44 asthe controlling unit. It ensures that the density and thus the coppercontent of the etching medium in the electrolytic cell 5a is kept at apredetermined value. This takes place, as described above, by startingor stopping the pump 29a. Once more this control system is completelyisolated from the first control system, which contains the etchingmachine 1, since the electrolytic cell 5a can deliver depleted etchingmedium to the buffer tank 3 independently of the instantaneousrequirement. Likewise, enriched etching medium can always be suppliedfrom the buffer tank 4 to the electrolytic cell 5a according to therequirements of the control circuit governing this, irrespective ofwhether etching medium just enriched there is or is not presentaccording to the state in the etching machine 1.

Due to the afore-described isolation of the two control systems, it ispossible to regulate and keep constant the density of the etching mediumat the actually critical point, namely in the etching machine 1, withgreater precision than this could be achieved when using solely onecontrol system, which covers both the etching machine 1 as well as theelectrolytic cell 5a. However, the two buffer tanks 3 and 4 can be usedin a further advantageous manner, details of which will now be given:

The previous description was given on the assumption (as alreadymentioned above), that the quantity of enriched etching medium removedfrom the sump 14 of the etching machine 1 by the pump 26 over apredetermined time is relatively small. The pump 29a, which supplies theetching medium from the buffer tank 4 to the first electrolytic cell 5a,is in this case capable of once more pumping out all the etching mediumintroduced into the buffer tank 4 from the etching machine 1. The levelof the filling height in the buffer tank 4 remains below the height ofthe opening of the line 28a in the buffer tank 4, thus below the heightdesignated by the reference N₁ in the drawing. However, if a greaterquantity of etching medium occurs, integrated over the predeterminedtime, then finally the delivery capacity of the pump 29a alone is nolonger sufficient for discharging this etching medium from the buffertank 4. The filling height in the buffer tank 4 rises until the liquidlevel reaches the height N₂. This is the height at which the line 28bopens into the buffer tank 4. Now the pump 29b also becomes operative,which supplies etching medium to the electrolytic cell 5b. Theelectrolytic cell 5b co-operates with the associated control circuit andthe storage tank 6b in the same way as was described above in detail forthe first electrolytic cell 5a and the corresponding storage tank 6a.The etching medium depleted by the electrolytic cell 5b passes by way ofthe overflow 34b into the line 35 and from there into the buffer tank 3.

If even the combined delivery capacity of the pumps 29a, 29b is nolonger sufficient for discharging the etching medium supplied to thebuffer tank 4 in the predetermined period of time, the filling height inthe buffer tank 4 rises above the value N₂, until finally the value N isreached. Located at this height is the opening point of the line 28c,which leads to the pump 29c and from there to the third electrolyticcell 5c. Now, in this way, the third electrolytic cell 5c also comesinto operation in the same way, as was described above the electrolyticcells 5a and 5b. Naturally, if required, further pumps 29 andelectrolytic cells 5 may be added.

In the embodiment illustrated in the drawing, the pumps 29a, 29b, 29cand thus the electrolytic cells 5a, 5b, 5c were operative at differentlevels of the filling height in the buffer tank 4 due to the fact thatthe opening points of the associated lines 28a, 28b, 28c lay atdifferent heights. Naturally it is also possible to locate all theseremoval lines 28a, 28b, 28c close to the bottom of the buffer tank 4 andto control the switching-on of the various pumps 29a, 29b, 29c bysuitable electrical level sensors.

The embodiment of the invention illustrated in the drawing represents amechanical realisation of a principle, which can also be implementedelectronically. The buffer tank 5 is basically nothing other than anintegrator, which integrates the quantity of etching medium suppliedthereto over a predetermined period of time. The levels N₁, N₂, N₃represent limit values of this integral. It is therefore also possibleto replace the buffer tank 4 by an electrical device, in which the samelogical functions are carried out. In detail this takes place asfollows:

A flow meter is located in the line 27, by which enriched etching mediumis removed from the sump 14 of the etching machine 1. The output signalof the flow meter is supplied to an electrical integrator, whichintegrates this output signal in each case over a predetermined periodof time. When the output signal of this integrator reaches one ofseveral limit values stored in a memory, then the path to a (further)electrolytic cell 5a, 5b, 5c respectively is released or blocked. Forthis purpose, a comparator is used, which compares the value of theoutput signal of the integrator with the limit values stored in thememory. The comparator may then bring the pumps 29a, 29b, 29c forexample into or out of operation in succession. The point at which thelines 28a, 28b, 28c combine and acquire a connection to the line 27,does not need to be located in a buffer tank 4. Of course this ispossible, if the above-described isolating effect of the buffer tanks 3,4 between the control systems of the etching machine 1 and electrolyticcells 5a, 5b, 5c is to be utilised.

In known etching installations, the operating times of the etchingmachine 1 and of the electrolytic cells 5a, 5b, 5c used for theregeneration of the etching medium are identical daily. If, as describedabove, buffer tanks 3 and 4 are used, the operating times may be keptdifferent. In this way, a smaller capacity of the electrolytic cells 5a,5b, 5c is adequate; it no longer needs to be adapted to the peakrequirement of the etching machine 1.

An example

The etching machine 1 is designed so that it etches away 9 kg Cu perhour, i.e. 72 kg in an 8-hour working day. In order to recover the samequantity of copper in the electrolytic cells 5a, 5b, 5c, provided thatthey work for 24 hours a day, a capacity of 3 kg Cu/hour is sufficientfor this. If the depletion in the electrolytic cells amounts to 50 gCu/l, this means that 1440 l etching medium must be buffered. However,since approximately 480 l are recovered from the electrolytic cells 5a,5b, 5c in the 8 hours of the operating time of the etching machine 1,each buffer tank 3, 4 must contain approximately 1000 l.

The continuous method of operation of the electrolytic cells 5a, 5b, 5cpossible when using the buffer tanks 3, 4, not only reduces theexpenditure for apparatus of the entire etching installation; inaddition it improves the regeneration operation in the electrolyticcells 5a, 5b, 5c.

During the operation of each etching installation, but particularly whenusing buffer tanks 3, 4, liquid losses occur due to evaporation. Theseliquid losses must be compensated for. In the above-described etchinginstallation, this takes place as follows:

The levels in the buffer tanks 3, 4 are monitored continuously by levelsensors 49 or 50. The latter are connected to an electronicwater-control unit 51. By way of electrical leads, which are shown inbroken line in the drawing, the electronic water-control unit 51 againcontrols a first solenoid valve 52, which controls the fresh watersupply into the left-hand buffer tank 3 in the drawing, as well as asecond solenoid valve 53, which controls the fresh water supply to theright-hand buffer tank 4 in the drawing.

Due to the above-described method of operation of the etching machine 1and of the electrolytic cells 5a, 5b, 5c, it is ensured that a constantfilling level is guaranteed therein without special measures. It istherefore solely still necessary to ensure that the sum of the levels inthe two buffer tanks 3, 4 likewise remains constant. This is exactly thepurpose of the water-control unit 51. If the latter ascertains a drop oflevels in the buffer tanks 3, 4 in such a way that the sum thereof fallsbelow a reference value, it opens the solenoid valves 52, 53 until thesum of the levels has once more reached the desired value. In order toobviate undesirable dilutions of the etching medium, which could disturbthe operation of the various control circuits, the addition of water toeach of the buffer tanks 3, 4 takes place in proportion to therespective level in this tank. Thus, for example, if the level in theleft-hand buffer tank 3 in the drawing is twice as high as in theright-hand buffer tank 4 in the drawing, then the addition of water bythe water-control unit 51 takes place in such a way that twice as muchwater is supplied to the left-hand buffer tank 3 as to the right-handbuffer tank 4.

I claim:
 1. Installation for etching objects, in particular printedcircuit boards, witha) at least one etching machine, in which metal isetched from the objects, the etching medium being enriched with metal;b) at least one electrolytic cell, in which enriched etching medium isdepleted; c) at least one electronic control circuit, which controls theexchange of etching medium between the etching machine and theelectrolytic cell so that the density of the etching medium in theetching machine is substantially constant,characterised by d) at leastone further electrolytic cell (5b, 5c), which is connected in parallelwith the first electrolytic cell (5a); e) a device (4, 28a, 28b, 28c,29a, 29b, 29c), whichea) measures the quantity of the enriched etchingmedium removed from the etching machine (1) and integrates it over acertain period of time; eb) in which as many limit values of varyingamount of the integral are stored, as there are electrolytic cells (5a,5b, 5c) in the installation; ec) on exceeding each limit value of theintegral a (further) electrolytic cell (5a, 5b, 5c) is set in operationor on falling below each limit value of the quantity integral a(further) electrolytic cell (5a, 5b, 5c) is put out of operation,so thatthe more electrolytic cells (5a, 5b, 5c) are operating, the moreenriched etching medium is removed from the etching machine (1) in thepredetermined time.
 2. Etching installation according to claim 1,characterised in that the device named in e) comprises a buffer tank(4), into which the enriched etching medium removed from the etchingmachine (1) is introduced, the buffer tank (4) being connectedrespectively by way of a line (28a, 28b, 28c), in which a pump (29a,29b, 29c) is located, to each electrolytic cell (5a, 5b, 5c) and anotherlevel (N₁, N₂, N₃) of the filling height in the buffer tank (4) isassociated with each pump (29a, 29b, 29c) so that it is effective solelyafter exceeding this level(N₁, N₂, N₃).
 3. Etching installationaccording to claim 2, characterised in that level sensors are provided,which monitor the reaching of the various levels (N₁, N₂, N₃) of thefilling height in the buffer tank (4) and in response to this set thecorresponding pumps (29a, 29b, 29c) in or out of operation.
 4. Etchinginstallation according to claim 2, characterised in that the lines (28a,28b, 28c), by which the etching medium is removed from the buffer tank(4), terminate at the height of the various levels (N₁, N₂, N₃) of thefilling heights and that the pumps (29a, 29b, 29c) are constantly inoperation.
 5. Etching installation according to claim 2 characterised inthat a buffer tank (3) is likewise located in the line (35), by whichthe depleted etching medium is conveyed backfrom the electrolytic cells(5a, 5b, 5c) to the etching machine (1).
 6. Etching installationaccording to claim 5, characterised in that the etching medium suppliedto one buffer tank (4) from the etching machine (1) and the etchingmedium supplied to the etching machine (1) from the other buffer tank(3) are guided by way of a heat exchanger (25).
 7. Etching installationaccording to claim 5 characterised in that a water control unit (51) isprovided, which keeps the sum of the filling heights in the varioussumps (14, 32), containers (16, 38) and tanks (3, 4) of the etchinginstallation constant by adding fresh water.
 8. Etching installationaccording to claim 7, characterised in thata) the filling heights in thebuffer tanks (3, 4) are monitored by level sensors (49, 50), which areconnected to the water control unit (51); b) the sum of the fillingheights in the buffer tanks (3, 4) is kept constant by adding freshwater; c) the filling heights in the other parts of the installation arekept constant independent of the addition of fresh water.
 9. Etchinginstallation according to claim 8, characterised in that the addition offresh water to each of the buffer tanks (3, 4) takes place in proportionto the filling heights of these buffer tanks (3, 4).
 10. Etchinginstallation according to claim 1, characterised in that the devicementioned in e) is constructed electrically and comprises:ed) a flowmeter; ef) an integrator, which integrates the output signal of the flowmeter over the predetermined time; eg) a memory, in which the variouslimit values of the integral are stored; eh) a comparator, whichcompares the output signal of the integrator with the limit valuesstored in the memory and on reaching one of these limit values sets theassociated electrolytic cell (5a, 5b, 5c) in or out of operation.